"Too many concurrent SMTP connections; please try again later." ?
It means the server your sending through is busy and you probably don't have SMTP Authentication enabled.
Islandnet.com has a pool of connections reserved for customers and then the rest of the connections are available to others who have received email from us white listing the IP address for a short period.
Enabling SMTP Authentication and using your account credentials will make sure that you are allocated one of the reserved connections minimizing the possibility of seeing this message
Note: There will be a time when we make SMTP Authentication a requirement so we advise all customers enable this feature when sending mail through us.